There have been a few discussions around the net concerning how MS has accomplished their BC for 360 games. Not only is the X1 running 360 games but it is also running the 360 OS as well. If we think about how the X1 works, there are 3 virtual machines. One is the main VM that host the games and App VM. It appears with the 360 BC that MS can either change, swap or add another VM which they are doing with the 360 games. With this particular tech, what is stopping MS for adding, changing or swapping a PC VM.
When Windows 10 release, the X1 will be updated with its kernal. MS should be able to run a WIndows 10 PC VM just like they run the other OS and thus giving them access to PC games on the X1. At least that is my theory but the question is if possible will MS go down that route. One benefit is that for indie developers who are already making PC games, no need to developer for the X1 and the X1 gets the benefit of a huge library of games already running on the PC.